Improve DoxiaFilter document source log message

We can improve the log message for:

• DoxiaDocumentRenderer: the Doxia source path which has been used to render the output
• ReportDocumentRenderer: no change, but source may be null, take care of that
• SitePluginReportDocumentRenderer: will use report mojo info
• eveything else will try DocumentRenderingContext#generator or the class name, if the former is null